For some years, there has been extensive use of radio frequency (RF) channels to transport data packets, such as those used for communicating via the Internet (e.g., Internet Protocol (IP) packets). Such technologies have been used both in scenarios in which some or all of the nodes are stationary (e.g., to link a cellular base station to a regional center), as well as in scenarios in which some or all of the nodes are moving (e.g., for communication between aircraft). The simplest versions of these communications links employ omni-directional antennas because there is no need to correctly point such antennas in order to communicate. High performance versions, however, typically employ directional antennas because they provide higher quality (e.g., faster) communications links.
In recent years, there has been a growing interest in using optical (e.g., laser based) links instead of RF links. Laser beams are harder to correctly point than RF beams, however, because laser beams are typically much narrower than even highly directional RF beams. Once the laser beams are correctly pointed, however, they can provide much faster communications links than RF beams. For example, a freespace optical link might provide 10 Gigabits per second (10 Gbps) throughput, whereas an RF link of similar size and power might provide 500 Megabits per second (500 Mbps). Thus, in this example, the optical link is twenty times as fast as the RF link.
Optical links have significant drawbacks, however, even aside from the difficulty in accurately pointing them. First, they may experience momentary or prolonged outages due to obscurations in the atmosphere, such as dust, fog, clouds, or other particulates. Second, blooms of atmospheric turbulence may momentarily defocus or bend the beam so that it does not reach the receiver at sufficient power for correct reception. Third, when used on moving platforms (e.g., aircraft or spacecraft), the platform motion itself may induce short outages, such as when an airplane banks and a wing comes between the transmitter and receiver or when a spacecraft does not quite properly compensate for its own vibrations and hence mispoints its beam for a short while.